This exact question was one that opponents of Darwin challenged him
with - "What good is half a wing?" In order to get to the point of having
a wing as well developed as that of a dragonfly or butterfly, what
advantage did the half a wing have? Where did the nerves and muscles
come from to handle such a complex movement as flight.

The answer to the origin of the wing can be found by looking at the
primitive forms of insects. All animals, as they mature, show clearly
their evolutionary process, be it human or dragonfly. The larval form
of the dragonfly is an aquatic beast. Looking closely at it, one can
see gills rapidly beating away pushing oxygenated water over them.
It is currently accepted that this is the origin of nerves and muscles
that allow for control of a wing. In some aquatic insects, the gills
also act as the method of movement, much like small oars.

The question of what good is half a wing still remains. Going from a small gill that can act as an oar and push an insect around under water has a long ways to go to become a dragonfly wing - or is it? The clue to what good half a wing is comes from water skimming insects that move about on the surface of the water held up by surface tension. Here, even a very small wing can prove an asset, giving the organism that extra push to move it faster than some other organism and allowing it to survive.